Autoformer vs Speaker impedance Curve

How fortunate are we in having two of the very best hi-fi amplifier designers/engineers working today (Ralph Karsten of Atma-Sphere and Roger Modjeski of Music Reference) sharing their knowledge with us here? That products of the quality they offer are made by not only such talented engineers, but also such nice guys, I am very grateful for. 

I note that this discussion has many questions concerning the use of an autotransformer with high damping solid state (voltage paradigm) amplifiers. I would say there is little to gain and a lot to lose using one. Here's why. This does not apply to the McIntosh amps and I previously stated their reasons for using autotransformers in their SS amps. 

First they place an almost zero DC resistance to the SS amplifier. That means a small offset voltage will create a large offset current which may be large enough to bring a torroid close to saturation. It takes very little DC to bother a non-gapped core. This is why tube amps should never have a torroid output transformer. 

Second, modern SS amps already have good current capability and low output impedance. They don't need any help

Third, good autoformers will do nothing to modify the wild impedance curve and reactive load that some speakers present. Some discussion of load line limiting in SS amps needs to enter here.

Heres what they will do. 

High output impedance OTL amps like the Atmasphere will benefit as their ideal load is typicaly higher than 8 ohms. My single pair OTL likes 64 ohms so I autoform it down to 16, 8 and 4 ohms and wrap 6db of feedback around the autoformer. 

All OTL amplifiers suffer from lack of current though there is plenty of voltage. Hey they are tubes... However the Futterman family has inherently low output impedance so the autoformer does not help with impedance but does exchange this excess voltage for improved current. A 2/1 autoformer will double the current available to the speaker. 

+1 Eric (bdp24).

We are privileged to have highly experienced and accomplished designers of outstanding audio electronics, such as Ralph and Roger, sharing their knowledge with us here.

Best regards,
-- Al 

@ramtubes
Hello Roger, welcome to the forum. Thanks for your posts!
We used to make an autoformer we called the Z-Music autoformer, which had taps for 4,3,2 and 1 ohm loads. With it, Steven Stone of TAS was able to listen to his Apogee Full Range loudspeakers with tubes for the first time. Although the Full Range had a very low impedance (not much more than 1 ohm) they were otherwise easy to drive as they had no serious phase angles and were reasonable efficiency.
I think you and I are on the same page here- if an amplifier is acting as a voltage source, it really doesn't matter if it has an autoformer in its output section, especially as in the case of the Mac, which includes the device in its feedback loop. If its a voltage paradigm device, it will make constant voltage into all (practical) loads by definition.